1. Field of the Invention
The present invention relates to a polarizer used in a liquid-crystal display device, and a method of producing the polarizer; and particularly to a polarizer which is excellent in environmental durability so as to cause little change in transmittance of light rays, degree of polarization, hue of an image, and so on, even in a severe environment such as under a high-temperature condition or a high-humidity condition.
2. Description of the Related Art
Liquid-crystal display devices have been used in gauges in personal computers, desk-top calculators, electronic clocks, word-processors, automobiles and other machines recently. The demand for polarizers with high polarization performance has increased along with the use of such liquid-crystal display devices. Since such liquid-crystal display devices have been particularly used in all fields recently, it is therefore necessary to assume that the liquid-crystal display devices may be used under severe conditions. There is consequently a demand for polarizers which are excellent in durability so as to cause little change in transmittance of light rays, degree of polarization, hue of an image, and so on, even under a high-temperature condition or a high-humidity condition.
As a polarizer of this type, in the background art, generally used is a polarizer constituted by a polarizing element made of a polyvinyl alcohol (PVA) film containing iodine or dichromatic dye adsorbed thereto; and a pair of protective films each made of a triacetyl cellulose (TAC) film or the like and disposed on opposite, front and rear surfaces of the polarizing element so that the polarizing element is held between the pair of protective films, because of its relatively inexpensive cost and excellent polarization performance.
That is, the background-art polarizer is produced by a method including the steps of: dyeing a PVA film with a dichromatic iodine or dichromatic dye; crosslinking the PVA film with boric acid, diborate, or the like; orienting the PVA film uniaxially (in which the dyeing, crosslinking and orienting steps need not be performed separately, that is, maybe performed simultaneously, and the sequence of the respective steps is not particularly limited to the aforementioned sequence); drying the PVA film; and sticking the PVA film to protective films such as TAC films. Since the TAC films are high in moisture permeability and moisture absorptivity, there has however arisen a problem that superfluous moisture enters the polarizer to thereby deteriorate the characteristic of the polarizer in the outdoor or on-vehicle field requiring high thermal reliability under a high-humidity or high-temperature condition. In such a circumstance, examination has been made into the point that transparent films low in moisture permeability and moisture absorptivity are used as protective layers in a polarizer which uses a polyvinyl alcohol film containing iodine or dichromatic dye adsorbed thereto as a polarizing element (hereinafter abbreviated to “PVA polarizing element”) (Unexamined Japanese Patent Publications No. Hei. 6-51117, 7-77608 and 11-142645).
However, since the PVA polarizing element is hydrophilic, the polarizing element per se has high hygroscopicity originally. If a measure that simply films low in moisture permeability or moisture absorptivity are used as protective films as described above is taken, penetration of moisture exhaled from the PVA polarizing element is prevented so that the inside of the polarizer per se located in a high-temperature environment or the like turns into a high-temperature high-humidity state. As a result, the quantities of change in transmittance of light rays (hereinafter simply referred to as transmittance), degree of polarization, and so on, become large, so that the reliability of the polarizer is lowered.